Relay-regulating circuits



April 14, 10.931- R. H. LINDSAY 1,800,304

RELAY REGULATING CIRCUITS Filed July 6, 1928 3 Sheets-Sheet l Q INVENTOIQ BY Elimaiuy ATTORNEY April 14, 1931. R UNDSAY 1,800,304

KELAY REGULATING CITBCUITS Filed July 6, 1928 3 Sheets-Sheet 2 INVENTORlfifllindug ATTORNEY April 14, 1931. R. H LINDSAY RELAY REGULATINGCIRCUITS Filed July 6, 1928 5 Sheets-Sheet 3 INVENTOR. BY Ella ahaATTORNEY Patented Apr. 14, 1931 UNITED STATES PATENT OFFICE RUSSELL H.LINDSAY, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO AMERICAN TELE- PHONEAND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK RELAY-REGULATINGCIRCUITS Application filed July 6, 1928.

This invention relates to regulating circuits for telephone systems andfor signaling systems generally, and particularl to regulating circuitsemploying trams 0 re- 5 lays. v

In this invention there will be described regulating circuits consistingprincipally of trains of relays whose purpose it is to correct forvariations in the magnitude of alternating current flowing in atelephone or signaling circuit. In telephone systems or in signalingsystems generally which are suitable for the transmission of voicefrequency signals, a regulating circuit of the above mentioned type isdeemed practically necessary because in such systems there aresubstantial variations in attenuation in the transmitting media, dueparticularly to changes in the conditions of the weather. A suitable,regulating circuit may preferably be employed to correct for suchvariations in attenuation in order to maintain the transmissionequivalent of the transmitting medium highly constant. A specificapplication of the various regulating circuits to be describedhereinafter may be made in connection with an automatic pilot channelfor a carrier system, because at the present time there is a need forequipment which will make automatic corrections in the transmissionequivalent of the circuit.

While this invention will be pointed out with particularity in theappended claims, the invention itself, both as to its objects andfeatures, will be better understood when read in connection with theaccompanying drawing, in which Figure 1 shows a train of pairs ofstepping relays the operation of which is first described independentlyof its relation to a complete regulating circuit of the type shown, forexample, in Fig. 3 of the drawing, and in which Fig. 2 shows a similartrain of pairs of relays which are actuated by different operatingcircuits.

Referring to Fig. 1 of the drawing, there is shown a pair of relaytrains, there being two relays for each step, one in each train. RelaysA and B comprise the first step; relays A and B the second, and so on,the number of relays employed depending upon Serial No. 290,850.

the number of ste s there are to be in the system. Each of t ese relaysincludes three windings, two of which are designated a: and y and whichare wound inductively, and a third designated a wound non-inductively.Each relay of the upper train, A A A etc., has four armatures, the firsttwo of which are mechanically coupled by means of a bar of insulation,the first of these closing a contact 1 when the relay is deenergized andthe second closing a contact 2 when the relay becomes energized. Thesecond pair of armatures of each relay of the upper train is alsomechanicall coupled by a simi-- lar bar of insulation, t ese armaturesclosing contacts 3 and 4 when the relayis energized, the second of theseclosing a contact 5 when the relay becomes deenergized. Each relay ofthe lower train, such as B B B etc, includes three armatures two ofwhich close contacts 1 and 2 when each relay becomes energized, thethird closing a contact 3 when the relay becomes deenergized.

Both relay trains are under the control of two main relays F and G,which are under the controlof switches or ke s L and M, respectively.One step inclu ing a relay of t e upper train and a corresponding relayof the lower train may be completed by the closing and opening of one ofthe switches, switch M being responsible for stepping the relay trainsup in ascending numerical order by successively operating relays A B A Betc., and switch L being res onsible for stepping the same relay trainown in descending numerical order by releasing relays A B A B etc. y

In Fig. 1, the circuit arrangement is shown in the condition in whichsteps 1 and 2 are operated. In other words, relays A B and A B areoperated. Suppose that switch M then is closed. Current will then flowfrom battery IV through the winding of relay G and the contacts ofswitch M, thereby operating relay G. The armature of relay G will beattracted to open contact 1 and to close contact 2. Current will thenflow from battery W through winding a: of relay A the interconnectingcircuit including the armature and contact 2 of relay G, armature andcontact 1 of relay B winding 00 of relay A and conductor h. Relay A willthen operate and become locked by the closure of its contact 3, thelocking circuit including battery W, conductor h, windings y and 2 ofrelay A armature and contact 3 of relay A and conductor is.

Suppose that switch M is now opened. Relay G will be released,itsarmature closing contact 1'. 7 Current willthen flow from batteryW'through winding 00 of relay B causing relay B to operate, theinterconnecting circuit including battery WV, conductor 71., winding a:of relay B the armature and contact 2 of relay A and the armature andcontact 1 of relay G. The closure of contact 2 of relay B will causethat relay to become locked, the locking circuit including battery W,conductor h, windings y and z of relay B armature and contact '2 ofrelay B and conductor One step upward is thus completed by the operationof relays A and B upon the closing and opening of switch M, the closingof swith M causing relay A to operate and the opening of that switchcausing relay B to operate. Upon closing switch M again, a relay may beoperated, and upon opening this switch, a corresponding relay in thelower train, relay B is o'pe'rated,fthus completing another step, and soon. It will be seen,'therefore, that the relays step up in the order A BA B and so on, and that it is impossible to operate any relay in eithertrain until the preceding relay in the above order has been operate Inthe circuit arrangement shown in Fig. 1, in which steps 1 and '2 areshown in their operated conditions, suppose that switch L is closed.Relay F then operates, current flowing from battery WV through thewinding of relay F and the contacts of switch L.

V and non-inductive windings y and 2, respectively, of relay A The otherterminal of winding y of relay A is connected to the positiveterminal ofbattery lV through conductor 7L. Accordingly, the inductive winding' 1of relay'A 'bec'omes short-circuited,

. whereupon relay A becomes released. The

non-inductive winding 2 of relay A is then connected to'batteryW toprevent excessive current from flowing through the armature and contact3 of relay A during the short interval of time required for relay A torelease, the interconnecting circuit including battery V, armature andcontact 2 of relay Ffarrnature and Contact?) ofcrelay B noninductivewinding 2 of relay A armature and contact 3 of relay A and conductor is.

relay B through conductor 71.. Accordingly,

inductive winding 3 of relay B is shortcircuited and relay B releases.

Thus, one step down is completed, releasing relays A and B upon oneclosing and one opening of switch L, relay A being released when switchL is closed, and relay B being released when switch L is opened. Ifswitch L should be closed again, relay A will release, and when switch Lis opened again, relay'l? will release. It will be seen, therefore, thatthe relays of the trains shown in Fig. 1 step down in the order A B andA B and that it is impossible to release any relay in either train untilthe preceding relay in the established descending order has drawing, inwhich the switching event is the first in thecourse of taking a stepeither up or down. 1

One armature of each A relay moves between contacts 4 and 5, the fixedterminal of this armature being connected to a conductor 6, whilecontacts 5 and 4 are connected to conductors f and g. 7 These conductorsof the various A relays lead to the Various switching relays, to besubsequently de scribed. p v

Fig. 2 shows thesame pair of relay trains as is shown in Fig. 1,operated to step'these relay trains downwardly through conductors a andb and to step upwardly through conductors c and (Z. The operatingcircuit is somewhat different, however, being arranged so that as soonas the switch V is closed the trains will 'start to step up and willcontinue to do so, with a certain time interval between steps, untilswitch V is opened. Similarly, if a switch U is operated, the trainswill step down in a similar manner until switch Uisopened. This will bemore fully described hereinafter;

Relays P P and P and relays N N n, y

its complete operation'or release, and that the time required for itscomplete operation or release may be controlled in a manner well knownin the art.

Relay P is under the control of relay S and switch V, and relay N isunder the control of relay R and switch U. Vhen relay S is unoperated,relay P is operated, due to the flow of current from battery 7 throughthe winding of relay P and the armature and contact of relay S. Relay Pis also operated due to the flow of current from battery W through thewinding of relay P and armature and contact 1 of relay P Relay P is alsooperated, current flowing from battery W through the winding of re-' layP and the armature and contact of relay P Relays N N and N are similarto relays P P andP and are under the control of relay R and switch U,and relays N N and N are. also operated by similar currents frombatteryW. I

When switch V is closed, relay S is operated, due to the flow of currentfrom battery W through switch V andthrough the winding ofrelay S. V Thearmature of relay S opens the circuit of its contact, causing relay P torelease, one of the armatures of relay P moving from contact 2 tocontact 3 after a short periodv of time. Then, the negative terminal ofbattery W becomes connected to conductor c'through the armature andcontact 3 of relay P Accordingly, relay A becomes operated in thestepping up process in a manner similar to that described in connectionwith Fig. 1. It will be clear that current will flow from battery Wthrough winding w of relay A and through conductor h. The release ofrelay P soon results in the release of relay P because the circuit inseries with the winding of relay P becomes opened at contact- 1 of relayP Thereafter, relay P will also become released due to the opening ofthe circuit in series with its winding at the contact of relay P It willbe clearly understood that, while relays P P and P are slow actingrelays and provide a delay in their operation of a substantial timeinter- I val, this interval may be increased by adding relays similar torelay P as indicated in the drawing.

The release of relay P causes the operation of relay P again,' currentthen flowing from battery W through the winding of relay P and throughthe armature and contact 1 of relay P The operation of relay P willlater cause the operation of relay P and the operation of relay P willcause the operation of relay P in the manner already describedhereinabove. Thus, one complete cycle ofevents for relays P P and P iscompleted, this cycle including the successive release of these relaysand their successive operation thereafter.

When relays P and P become both operated at the same time at thecompletion of this cycle, the negative terminal of battery W becomesconnected to conductor d through the armature and contact 2 of relay Pand through the armature and contact 2 of relay P This will cause theoperation of relay B in the manner described in connection with Fig. 1of the drawing, relay B becoming operated because current flows frombattery 1V through the armature and. contact 2 of relay A throughwinding :2: of relay B and through conductor it. Thus, one complete,step is completed, this step being started upon the release of relay Pand completed upon the operation of relay P3.

If switch V still is closed and relay S operated, the operation of relayP causing the opening of its contact 1, will result in a second releaseof relay P and the starting and completion of a second cycle, in thesame manner as already described hereinabove. Upon the release of relayP relay A will become operated, and when all three relays, P P and P areagain operated, relay B will become operated. As long as switch Vremains closed this stepping action will continue. If at any time in thecourse of a cycle switch V is opened and relay S is therefore released,the stepping action of the relay trains will be stopped when theparticular step .in progress at the time, including one relay in theupper train and a corresponding relay in the lower train, is completed.In other words, the cycle of the events with respect to relays P P and Pwill become completed in due course by the return of these relays totheir normally operated condition. If switch V is opened and relay Sreleased at a time when relay P is operated there will be nointerference with the progress of the cycle of events with respect torelays P P and P and the cycle will be completed in the normal timeinterval. If switch V is opened and relay S therefore released at a timewhen relay P is released, the cycle not having been completed, therewill be no interference with the progress of the cycle except that relayP will become operated at a somewhat earlier time, which will result inan earlier completion of the cycle of events.

Relays N N 2 and N maybe the same as relays P P and P and, as statedhereinabove, are under the control of switch U and relay R. Relays N Nand N control the connection of the positive terminal of battery Wbetween two conductors, a and b, so that the two relay trains may bestepped down in the manner described hereinabove in connection with Fig.1 of the drawing.

Fig. 3 shows a relay regulating circuit complete except for theapparatus required forthecontrolimechanisJnactuating"switches Land-M.Ashas. alreadybeenstated, the: chief purpose of this: circuit.isitozcorrectil for variations'in alternating: current which;

:inay: flow in a circuit... A- portion-of theacir cuit. transmittingsuch l current is shownzbe-i tween Z and min Fig; 3. The input at Z may:varyr'between certain limits, and it is. desired to-maintainthe outputat we automatically. within. smaller limits. This; may: be donebyfcutting into or out of the circuit; one. or: more artificial lines,as shown, and these; linesmay be provided-in. elements of,.for example,-I, 2; 4' and 8' transmission units (TU) of attenuation. 7

When the input at Z varies to; such an ex tent as to require adjustmentof the artil.

ficial line pads in order to keep the-output atm within certainprescribed 'limits,the circuit may'beimade to recognize this .cond-itionby means of" the control mechanism which may pick. off; through aimpedance transformer T',.a' small part. of theoutput current.. Thiscontrol mechanismwillintercon'nect: the secondary winding of transformerT withswitches L and. M in any: Well known manner, thus permitting the:closingan'd' opening of switches-L and M,

' asdesired. One form of such controlmechany cause the, effect of whichitisdesired-ito.

counteract. In the .case. of. atransmission circuit suitable for: thetransmission of tele phone messages; the most common. causes; arise fromvariations in Weather conditions; as. stated, hereinabove. Analternating: cur rent of suitable frequency and of substan-. tiallyconstant amplitude-isoften transmit tedv over :a. line for the purposeof. actuating the control mechanism, Which wilLIrespon'd to. changesinthe magnitude of thiscurrent caused-by changes in Weather. Since in thecase of 'atelephone circuit'there are trans mitted at the same time.other currents of; widely different and rapidly changing fre quenciesand magnitudes, it is, then necessaryfor the control mechanism to besensitive only to the current .of the. frequency trans-s mitted forregulating purposes. A. required sensitivity may be obtained by tuningthe input circuit ofthe control mechanism ii. e.,

nsnogece in": Fig: 3; of; the: drawing thacircuitt ofi'the'a secondary:Winding f transformer- T; .tolzthe: regulating" frequency by' means: ofvariie able condenser, or the like.

The; stepping. relay. trains; consisting: of

relays-A to: A andQofrelaysrB} 1103B may: be the same-astliOSG.LdGSCfIlbEdfalIIjiCOILHGCtiZOIlZ withzFig; l and may be: operatedthrough,

conductors c and. (Z3 forstepping these relay trains up. and operatedthrough conductors: wand b for stepping these relaygtrains'downq Theserelayxtrains control I switching relays :I',

J and1.K, .which. may control; artificial. line pads exhibitingattenuations: corresponding-j to 2,14and 8' transmission; units,respectively:

The relation between relay 'H' andrelays- I J and will be brought out inthedescrip tion-hereinafter following. Relays I; J and' K may be-employed tointerpose. any desired combination ofthe artificial linepads to provide a-change in attenuation. of two trans mission units perstep,-. as indicated in" the accompanying- Table-- I.

Step of Switching Total relay: relays oper attenuatrain. a (1 tion l 0 V0... 0. 1': I 2 2 I. 4,. 3; LJ: 6 K1 8; 5' 1,]! 10.. s; 1;]: 12 7 I, .T,K. 141- A fourth artificial line pad exhibiting; attenuationcorresponding to one transmission unit is "under the control ofswitching. relay H. Relay'His operated. by-a special relay circuitconsistingof relays C C D1,

B and, E, in: such; a way that; steps of one;

transmission unit. are, provided instead of; stepsoftwO transmission.units: Table llI' showshow relay H'afi'ects, the operation of,

Upon. examination of the, combinations shown in Table .II,-.it;will: be.notedthatrelay H. is alternately. operated; and .released'ias. the

value of. attenuation is. to. be successively leased a second time.

and that whether or not it must change its step is determined bythedirection in which the change of attenuation need take place.

Relays C C D D 'and E are shown released. Suppose relay E is arbitrarilyoperated. Relays C and C will, therefore, operate, current flowing frombattery W through the winding of relay C2, windings y; and z of relay Carmature andcontact l of relay D and armature and contact 2 of relay E.Upon the closure of contact 3 of relay Ci, relays C and C; will becomelocked. Suppose relay E becomes arbitrarily released. Relays D and Dwill become operated, current flowing from battery W' through thewinding of relay D windings 3 and z of relay D armature and contact 2 ofrelay C and armature and contact 1 of relay E. Upon the closure ofcontact 3 of relay D relays D and D will become locked.

Suppose relay E becomes operated a second time. Then, the positiveterminal of battery W becomes connected to the junction of the inductiveand. non-inductive windings fl and 20f relay C througharmature andcontact 3 of relay E and armature and contact 2 of relay D The positiveterminal of battery W is also connected to the winding of relay G whichis in series with the inductive winding g of relay C Accordingly.winding 2 of relay C and the winding of relay C become short-circuitedand relays C and C both release. The non-inductive winding of relay Cserves to prevent excessive current from flowing from battery W duringthe short interval of time required for contact 310i relay C to becomeopened.

Lastly, suppose that relay E becomes re- 7 Then, the positive terminalof battery W will be connected to thejunction of the inductive andnon-inductive windings 2 and z of relay-D through the armature andcontact 4 of relay E and the armature and contact 1 of relay C Thepositive terminal ofbattery W is also connected to the winding of relayD which is in series with the inductive winding 3/ of relay DAccordingly, Winding g of relay D and the winding of relay D will becomeshort-circuited and relays D and D will be released, the non-inductivewinding .2 of relay D preventing excessive current from flowing frombattery W during the short interval of time required for contact 30f Dto become opened.

Table III shows a cycle of events passed by relays C C D and D as relayE is successively operated and released. It will be apparent that upon athird operation of relay E'this cycle of events starts to repeat.

Gondz'tz'on of relays E Cl, C: 1, 1

1st operation Operated. Released. 1st release Operated. Operated. 2ndoperation Released. Operated. 2nd release Released. Released. 3rdoperation Operated. Released.

Relay E is under the control of both relays F and G. \Vhen either ofthese relays operates relay E operates, and when it releases relay Ereleases, except, however, that relay E remains rcleased and unaffectedby relay G when the main transmission channel is shunted by all of theelements of attenuation, and that relay E remains released andunaffected by relay F when the transmission channel is shunted by noneof the elements of attenuation. These circuits affected by relay E willbe described more fully hereinafter.

The circuit shown in Fig. 3 of the drawing is shown in the condition inwhich the 'main transmission circuit or channel is be increased. Thecontrol mechanism (connecting the secondary of transformer T andswitches L and M) will start to close and open switch M periodically. Achange in attenuation from one corresponding to four transmission unitsto one corresponding to five transmission units requires that relay Hshall be operated while the relay trains remain on the second step, aswill be apparent from Table II. When switch M closes relay G operates,current flowing from battery W through the winding of relay G and switchM. Relay E then operates, current flowing from battery IV through thewinding of relay E, armature and contact 4: of relay B and armature andcontact 3 of relay G. Upon the operation of relay E relays G and Coperate, as will be apparent upon inspection of Table III. Relay H willthen operate, current flowing from battery W through the winding ofrelay H and armature and contact 2 of relay C Two resistances will thenbe connected in series with the transmission circuit, and one in shunttherewith. These resistances, representing an attenuation to the extentof one transmission unit, increase the total attenui;by1 the operationofzrelay .J. )The :clos'ure ofcontact :2 :ofrelay G willaccomplishnothing at this time becauselcontact 2 of relay D is open;Relay trains'including relays "A, to A and relays "B to B remainundisturbed, andthere is no change in their step.

When switch M is opened relay G releases, thereby releasing relay 'E."Gonse quently, relays D and D operate. The closure ofcontact2 of relayDhas noieflect on the relay trains because contact 3 of re- @sion unitsto :SiX transmission :units.

wlay C is open. Thus, after closing and opening switch JM the totalattenuation presented to the transmission circuit ,is changed from ,fourtransmission units to five transmission units. 7

If the output at'm 'thenhbecomes normal,

the control mechanism :will cease to -operate..

31f further attenuation is required to :bring about a substantiallynormalcondition at m, the control mechanism will operate .to -glose switch.M;aga-in. Upon inspection .of {Fable .to become released' Upon theiopening of .lcontact lf relay =C relay H'wi-ll -be released. Therelease of relay Q also causes the connection of the negative terminal"of battery -W- to conductor- 0 through the armature and contactj2-ofrelay G, armaturerand contact .3 of relay G 2 and v armature .and-.contact 12 0f ;-r,el;ay D ,Relay A of'fthe @upper train will then;operate, starting 3a :step up- Ward. 7 Relay I will then operate,current :flowing from-battery .lvthroughthe wind- ;ingeof relayI,anmatureandcontact A of relay A armature rand contactoio-f relay A andarmature and contact 4 of relay A Acc rdingly, the attenuation will beincreased by one transmission unit. *U pon vthe opening of switch .JMrelay B will be released, and 'thejrelease' of relay will cause relays:Di. and D to release. The ..=re-

lease of relay G will cause the-connection of the negative terminal ofbattery W :to conductor d through armature :and contact 11 ofrelay G.Relay B ,;in theIlower-train, will-then operate. If anattenuation-corresponding to six transmission units is sufii- ,cient tobring about a substantiallynormal output: at m2, the control mechanismwillstop operating and switch M will remain open. 7, Steps downward inthe -relay trains to bring about aulo-wer attenuationo-izthetransmissioncircuit, whenever'znecessary, are ac- 'eQmplisheClin a similar manner,switch L, .IlQtrSWitCh M, being-then operated by -=the controlmechanism. It should .be noted that, :uponvthe closure of switch -Lrelays]? .be-

comes goperated by the ,zflow of current :throngh its ":.w:indi'ng irom:battery rand .;.throngh switchllggairdzthatirelaysEibecomes operated,acurrentiflowing :from battery through the winding tofxr'elay 5E,armature :and .contact A of ;relay B --and;armatureand contact?),ofzrelayvrF. In other :respectsthe circuits completed .in stepping-.the attenuation downward will beapparent from ithe description givenhereinahove.

.Consi'deration "will now ihei given ;to the conditions oflthe circuitwhen theimaximum or the minimum zo'f attenuationiis provided by theregulating system. Jfithe aattenuation has been ,steppedup.toitsimaximum, i. e.', 15 transmission units,.all of theiswitching,relays, .i. e, relays EH, .1, J and become operated. If ;theattenuation presented to the ztransmissionwcircuitjis inslifli- :cientto ,properly reduce :;the .out .ut lexrl atom, :the control mechanismw-;to periodically .close and .open iswitchfM, resultingin aperiodicoperation ldfl'elay iE. lR-elay 11 will then he alternately released andoperated, continually .c'hargging 'the eattenuation from 14 transmissionunits 170115,- andhack again to 14. .Such a condition is undesirable,and means have "been pro- Vided'to eliminate that condition. Accord-'ingl y,cmeans are providedto prevent the loperation of relay 'Eflbyswitch'M when both relays Byand'H are operated. T'wo paraL Ilel.circuits, one including 'eontact' of:relay 1B and the other including'con.tact2 of relay are inserted ibetween contact gdf relay. G an'cl'thewinding of relayEfboth of these "circuits being open when frelays B-and"I-Ihave both operated;

In connection with the presentation o'f' the minimum =of attenuationfor-the transmission circuit, :it is "necessary to i prevent anyperiodicchange between. zero and- :one' -transcontinue mission unit bythe 'ioperati'on of irelay Inaa similar manner, relay :Eyis preventedfrom being op erated-byswitch-L Whelle.b0th

relays 8 :and vHare released. iT-wo circuits 1,

are. then-connected in parallel relationship between contacta3 ofirelayaF and :the winidings.o. f relay,E, .one of thesecircuitsfincluding contact at ofrelayuB and the other con- It will :be"apparent that this may 'be iac 'compli'shed'by replacing switches L andand relays "F and "by the two group's of relays above mentioned, and bythe 'a-ss'ociated switches U and V and relays B and '8, also of Fig. 2.Relays ,N wand ,=P -av.oultl c each z then require :an azdiiliti onalzbreak con-' .tact to perform the functions 10f contact?) 0n relays FeudItwilldoe understood that the arrangement of Fig. 3 is thus madesuitable not only for the type of control mechanism which, whenactuated, periodically closes and opens a switch or key such as L or M,but also for the type of control mechanism which, when actuated, closesand holds closed a switch or key such as U or V in Fig. 2. Furthermore,over-correction of the attenuation caused by possible slow response of acontrol mechanism such as the latter, may be overcome by the addition ofas many slow-acting relays as necessary in the relay groups N N and Nand P P and P The arrangement shown in Fig. 3 of the drawing permits agreat reduction in the number of relays ordinarily required to changethe attenuation of the transmission circuit by steps. Here two relaytrains, each including seven relays, permit fifteen changes inattenuation instead of seven, as may be ordinarily expected. In otherwords, seven steps of a pair of relay trains including fourteen relays,are employed to cover a range of fifteen changes in attenuation, or onemore than twice as many steps as are provided by the relay trains.

It will be understood that the arrangement of Fig. 3 includes nomarginal, slowacting or slow-release relays and that it, therefore,possesses a high degree of reliability.

It will be understood that the requirements for the control mechanismare moderate in that there are substantially no restrictions upon themanner of operation of switches L and M except that these switches beopened and closed for intervals long enough to operate or release therelays concerned to bring about the proper stepping sequence, and thatboth of these switches must not be closed simultaneously.

It will be understood that, while this invention has been shown incertain particular arrangements merely for the purpose of illustration,the general principles of this invention may be applied to other andwidely varied organizations without departing from the spirit of theinvention and the scope of the appended claims.

What is claimed is:

1. In a gain control system, in combination, a plurality of relayscomprising a first train, a plurality of relays comprising a secondtrain, each relay in the second train corresponding to a particularrelay in the first train, two pairs of control relays, means extendingfrom said control relays to the Various relays of said first and secondtrains, whereby a relay in the first train and a corresponding relay inthe second train may be operated to change the gain of the system in onedirection, and means extending from said control relays to the variousrelays of said first and second trains, whereby a relay in the firsttrain and a corresponding relay in the second train may be released soas to change the gain of the system in the opposite direction.

2. The combination of a plurality of relays comprising a first train, aplurality of relays comprising a second train, two pairs of controlrelays, mean interconnecting said control relays with the relayscomprising first and second trains, whereby the relays of said trainsmay be progressively operated in one order, each relay of the firsttrain operating before the corresponding relay of the second train, andmeans interconnecting said control relays and the relays comprising saidfirst and second trains whereby the relays of said trains may beprogressively released in the opposite order, each relay of the firsttrain always being released before the corresponding relay of the secondtrain.

3. The combination of a plurality of relays comprising a first train, aplurality of relays comprising a second train, and a third group of slowrelease relays which are normally operated and which, when said thirdgroup of relays begin to release, cause the operation of a selectedrelay in the first train and which, when said third group of relays havebeen released and operated again, cause the operation of a correspondingrelay in the second train.

4. The combination of a plurality of relays comprising a first train, aplurality of relays comprising a second train, a third group of slowrelease relays which are successively released and then operated tooperate a selected relay of the first train and a corresponding relay ofthe second train, and a fourth group of slow release relays which aresuccessively released and operated to cause the release of a selectedrelay in the first train and a corresponding relay in the second train.

5. The combination of a plurality of relays comprising a first train, aplurality of relays comprising a second train, a third group of slowrelease relays which are normally operated, a fourth group of slowrelease relays, which are normally operated, means for progressivelyreleasing the relays of the third group to cause one of the relays ofthe first train to operate and step up, means for progressivelyoperating the relays of the third group to cause the corresponding relayin the second train to operate and similarly step up, means toprogressively release the fourth group of relays so as to cause a relayin the first train to release and step down, and means to progressivelyoperate the fourth group of relays so as to cause a corresponding relayin the second train to release and step down.

6. The combination of a plurality of re- 1.0 a V the secondtram, andmeans whereby the lays comprising a first train, a plurality of relayscomprising a. second train, each of the relays of the second traincorresponding to one of the relays of the first train, a first. pair ofcontrol relays, a second pair of control relays, means whereby the firstand second pairs of control relays may be successively released andoperated to operate one of the relays in the first train and then tooperate the corresponding relay of first and second pairs of control.relays may be successively operated and. released to release a relay ofthe first train and then to release the corresponding relay of thesecond train. 7

77.. In a gain control system, the combinationof a plurality of relayscomprising a first train, a plurality of relays comprising a secondtrain, each of the relays of the second train c rresponding to one ofthe relays of the first train, a transmission cir- "cuit, a plurality ofelements of attenuation associated with said transmission circuit, afirst pa r of control relays, a second pair of control relays, meanswhereby, the first and a second pairs of control relays may besuccessively operated and released to increase the number of elements ofattenuation connected to the transmission circuit, and means wherebythefi'rst and second pairs of control relays may be successivelyoperated and released to decrease the number of elements of attenuationconnected tothe transmission circuit. 7 v r a 8.'In a system for thecontrol of gain in transmission, the. combination of a plurality ofrelays comprising a first group, a plurality of relays comprising asecond group, each, relay. of one of said groups cor responding to oneof the relays of the other of said groups, a transmission circuit, aplurality of pads'of attenuation having magnitudes arranged in geometricprogression which may be connected to said transmission circuit,'aplurality of control relays which operate in succession, meansinterconnecting said control relays and the various relays of said firstand second groups to operate selectedrelays of said first group and thecorresponding-relays of the second group, means interconnecting saidcontrol relaysand the various relays of said first andsecond groups torelease selected relays of the first group and the corresponding' relaysof the second group, and means controlled by the relays of both groupsto change'the attenuation interposed by zsaid padsin said transmissioncircuit by increments which are in arithmetic progression.

In testimony whereof, I have signed my name to this specification this5th day of 7 July, 1928.

RUSSELL H. LINDSAY.

